The nuclear modulation effect in pulsed EPR will be used to study the chemical structure of imidazole in R and T state hemoglobins. Experiments will be performed on nitrosyl hemoglobin, with IHP, and a ferric hydroxide form from hemoglobin Kansas where it is suggested that a proton on the N-pros nitrogen of the proximal imidazole is believed to be lightly bound effecting the coupling of the imidazole to the iron. In this way, it is suggested that the affinity for oxygen is electronically mediated. BIBLIOGRAPHIC REFERENCES: Hoffman, B.M., Brautigan, L., Feinberg, B.A., Margoliash, E., Peisach, J. and Blumberg, W.E. Multiple low spin forms of the cytochrome c hemochrome: The EPR spectra of various eukaryotic and prokaryotic cytochromes c. J. Biol. Chem. 252, 574-582 (1977). Chevion, M., Traum, M.M., Peisach, J. and Blumberg, W.E. High resolution EPR studies of the fine structure of heme proteins; third harmonic detection approach. Biochim. Biophys. Acta. 490, 272-278 1977, in press.